Toward Single Electron Resolution Phonon Assisted Ionization Detectors Motivation DM and CENNS motivation. Detector concept. From CDMSlite to contact-free HV detectors Perspective Nader Mirabolfathi Texas A&M University CEvNS Workshop, Texas A&M Nov 2015

Two big question one solution 2

Target Light 1% energy Phonons 100% energy Ionization 10% energy WIMP CRESST II E phonons E ionization Background Signal ZEPLIN II and beyond XENON 1 T, LUX ArDM, WARP ZEPLIN I XMASS DEAP CLEAN CDMS, SuperCDMS EDELWEISS Event-by-event discrimination 3

Germanium at 300 K C V =3200 J/kg·K and  D =374 K  C V at 0.02 K = 5e-10 J/kg·K =3e6 keV/kg·K A thermometer attached to the absorber with  ~100: Therefore for a 1kg Ge absorber cooled down to 0.02 K,  T caused by a 10 keV deposit will be: ~3.3e-6 K Signal  I~ 40 nA Typical for SQUID Amplifiers 1 kg Ge T=0.02 K 2  V R (  ) T (K) Superconductor Transition Edge Sensor Noise~1pA/(Hz)^1/2 Very good resolution Calorimetery with large absorber mass: A back of the envelope calculation! 4

d3d3 d3>d3> d4d4 d4>d4> d2d2 d2>d2>d1d1 d1d1 Anharmonic decay rate   5 Elastic scattering rate   4 d: Mean free path 5

CDMS Principle Large Ge or Si crystals (~kg) cooled to: T < 0.04 K Measure recoil energy via Lattice vibrations (phonons) in Ge or Si Measure the Ionization. E- field: ~3V/cm Ionizing power (Ionization yield:Y) Y electron-recoil > Y nuclear-recoil Event-by-event discrimination Near surface events Electron recoil but poor charge collection Near geometrical boundaries h + e - R T Signal Charge deficit <5 micron<10 micron<20 micron Electron recoil Nuclear recoil 6

Luke-Neganov phonons Phonon noise doesn’t scale with the ionization bias: S/N  => In theory one can increase Bias to reach Poisson fluctuation limit: limitation: Ge Breakdown 7 Luke et al., Nucl. Inst. Meth. Phys. Res.A 289, 406 (1990) 3/27/2014N.Mirabolfathi-Texas A&M Phsyics

Low Noise Phonon readout Phonons among the lowest quantum excitations in condense matter detectors. Tremendous progress in low noise phonon readout down to fraction of eV for small calorimeters. Almost mass independent for athermal phonon measurement. Various options to handle backgrounds: event-by-event discrimination. Measure phonons directly or use Neganov-Luke effect to indirectly measure ionization down to e-h resolution.

CDMSlite: CDMS with phonon gain Use the Luke phonon amplification to indirectly measure ionization using very good phonon resolution. One iZIP (0.625 kg) used for this data ~70 kg.day exposure Impressive 14 eVee resolution for Vbias=69 Volts. 9 Limited to current leakage for V>70 Volts Or E> 24 Volts/cm Very low compared to standard 77K Ge detectors 3/27/2014N.Mirabolfathi-Texas A&M Phsyics

CDSMlite limitation: Breakdown?

Need to study breakdown! What causes the breakdown at such low fields? Impact ionization on impurities: – Ionized – Neutral Leakage through electrodes? Conduction over detector free surface: – Better surface treatment? – Common problem if surface damaged. J.P. McKelvey, Solid-state and semiconductor physics, Harper & Row, 1966.S.M. Sze, Semiconductor devices: physics and technology, 2nd Ed., Wiley, Injection through contact by tunneling Avalanche 11 3/27/2014N.Mirabolfathi-Texas A&M Phsyics

P-type Point Contact Ge 0.05 K PPC are common design for 77K Ge gamma spectrometers. Experiments using ultra pure Ge PPC: Majorana, CoGeNT, LBNL provided a prototype Majorana PPC. At UCB adhered a tungsten TES thermistor.

No Break down for V up to 400 Volts

E Field in PPC for V bias =400Volts 143/27/2014N.Mirabolfathi-Texas A&M Phsyics

CDMS symmetric contact geometry Ge W Al α-Si Use W TES (T C ~75 mK ) phonon sensors Both faces of the detector uniformly covered with sensors. Simultaneously measures athermal phonons and ionization. Design tailored for excellent background rejection. In CDMSlite mode only readout one face phonon sensors. The other face used for biasing.

CDMSlite symmetric contact geometry Ge W Al α-Si

CDMSlite symmetric contact geometry Ge W Al α-Si Vacuum Al electrode

High Voltage athermal phonon readout: contact free biasing One face of the detector processed similar to a CDMS II detector: fully covered by Four W based athermal phonon readouts. The other face of the detector left free Bias the detector through a gap ~ 0.5 mm TAMU Berkeley 18 Mirabolfathi et al. arXiv:

Contact free asymmetric results The phonon sensor always at ground potential. Observed a strong leakage polarity dependence. Significant leakage when biased negative. => Holes leak through the interface Requires further studies on contact properties.

Contact Free performance Mirabolfathi et al. arXiv:

Comparing Contact free and CDMSlite 2.5 cm Thick 1 cm Thick Mirabolfathi et al. arXiv:

Baseline Resolution 1 cm Thick 2.5 cm Thick => Given a fixed E critical × 2.5 V max => × 2.5 Phonon gain Using this concept in CDMSlite iZIP absorber geometry, we expect to achieve: σ < 2.8 eV ee Mirabolfathi et al. arXiv:

Conclusion Both low mass dark matter searches (e.g. CDMS) and coherent neutrino scattering experiments (e.g. MINERS) are striving for very low threshold detectors. CDMSlite already achieved unprecedented sensitivity for low mass WIMPs and is using the Neganov-Luke phonon amplification as detector principle. CDMSlite limitation of leakage current seems to originate form the particular electrode/absorber interface. Electrically insulating electrodes from the absorber can be a solution to reach very low threshold. For the very first step using vacuum as a perfect insulation, we achieved Luke amplification of ×50 and reached an RMS resolution of σ < 7 eV in a 0.25 kg Ge detector. We believe single electron resolution phonon mediated detectors are just couple devices away with exciting physics that they can deliver.

24 SuperCDMS LUX/Z Dark Matter Direct Detection Current Status

25 Low Mass Threshold Limited

Xe Ge Si Ar Ne Signal from a 100 GeV WIMP 26

Xe Ge Si Ar Ne 27 Signal from a 20 GeV WIMP

Xe Ge Si Ar Ne 28 Signal from a 10 GeV WIMP Background Integrate above E

SuperCDMS: Many Experiments in One 29

Understanding Low Voltage Breakdown in CDMSlite 30 UC Berkeley Nader Mirabolfathi, Kyle Sundqvist, Bruno Serfass, Arran Phips and Dana Faiez. LBNL Kai Vetter, Paul Luke, Ryan Martin and Mark Amman TAMU Rupak Mahapatra, Rusty Harris, Mark Platt, Andrew Jastram and James Phillips. High purity gamma spectroscopy 77K Ge detectors e.g. PPC at ~1000 V/cm A Collaborative effort formed between 3 institutions Proposal to UCOP INPAC-MRPI for R&D over 2 years in /27/2014N.Mirabolfathi-Texas A&M Phsyics